I'm putting a new FSA Alpha drive 3 ring crankset on my bike. The old bottom bracket was a 68x127 Shimano UN54. This is way to long and my chainline is way off. What measurment do I need to take to determine the correct spindle length? With the old BB and old crankset in the distance from the centerline of the seattube to the rings are 40mm,50mm, and 60mm. With the new crankset on the measurments are 50mm, 60mm, and 65mm. So I'm guessing that since the new crankset is out by about 10mm vs the old one I need a spindle length that is about 10mm shorter (so about a 118mm). Does this sound correct? It's a 7 speed if that makes any difference.
If your old small ring is 40mm from centerline, and the spindle is symetrical (equal length left/right), and you want to bring your new setup in 10mm, then the spindle should be 20mm shorter (10mm each side).